Book Name:Husn e Zan Ki Barkaten
Dear Islamic brothers! Every such thought which is determined from an apparent sign is known as assumption, and is also referred to as opinion. For example, seeing smoke from a distance and having the thought of a fire being present is known as assumption. (Mufridat Imam Raghib, p. 539)
Assumption has two types: One is good assumption and the second is bad assumption.
The meaning of good assumption is: ‘اِعْتِقَادُالْخَیْرِ وَالصَّلاحِ فِیْ حَقِّ الْمُسْلِمِیْنَ’, i.e., ‘Having certainty of goodness and uprightness regarding a Muslim.’ (Mirqat-ul-Mafatih, vol. 8, p. 779) For example, seeing someone exiting the Masjid at the exact time of Jama’at and assuming that he must not have Wudu or has another valid Shar’i excuse.
The ruling on having a good assumption
Maulana Sayyid Muhammad Na’eem-ud-Deen Muradabadi رَحْمَةُ الـلّٰـهِ عَلَيْه mentions the types of assumption and their rulings as follows: There are many types of assumption: One is Wajib which is to have a good opinion of Allah Almighty; one is Mustahab, which is to have a virtuous opinion of a righteous Muslim; one is forbidden and Haraam, which is to have a bad opinion of Allah Almighty and a believer; one is permitted, which is to have an opinion of an open sinner in accordance to the actions he performs. (Khaazin-ul-‘Irfan, Part 26, Surah Al-Hujurat, under the verse 12, p. 950)
Ameer-e-Ahl-e-Sunnat دَامَـتْ بَـرَكَـاتُـهُـمُ الْـعَـالِـيَـهْ states: There is no harm in having a good opinion, and there is no benefit in having a bad opinion.